A conventional bridge PFC circuit has many conduction devices therein and a large on-state loss, and is not suitable for application in scenarios of medium or large power. However, a bridgeless PFC circuit may reduce the on-state loss and improve the efficiency. As the market demand for a high efficiency and high power density power supply increases, the replacement of the conventional bridge PFC circuit with the bridgeless PFC circuit becomes a trend. FIG. 1 shows a topology of a bridgeless PFC circuit, which is a two-phase interleaved bridgeless PFC circuit.
An existing bridgeless PFC circuit generally adopts a critical mode (CRM) control method. That is, when an inductive current approaches zero, a switching component (for example, a metal-oxide-semiconductor field-effect transistor (MOSFET)) is turned off, the current continues to flow through a body diode of the switching component, and zero voltage switching (ZVS) is implemented depending on a reverse recovery current of the body diode of the switching component.
In the two-phase interleaved bridgeless PFC circuit shown in FIG. 1, two bridge arms work with a 180-degree phase difference between drives of the bridge arms. In the same way, if the bridgeless PFC circuit has three bridge arms, three bridge arms work with a 120-degree phase difference among drives of the bridge arms. A working principle of a single bridge arm (that is, a bridge arm connected to an inductor L1) is briefly introduced herein. In the positive half cycle of inputting an alternating current, a MOSFET Q2 acts as a main transistor. In the on time Ton of the MOSFET Q2, a current loop passes through the inductor L1, the MOSFET Q2, and a diode D2, and at this time, the inductor L1 stores energy. In the off time Toff of the MOSFET Q2, the current loop passes through the inductor L1, a MOSFET Q1, a capacitor C, and the diode D2, and at this time, the inductor L1 outputs energy. Similarly, in the negative half cycle of inputting an alternating current, the MOSFET Q1 acts as the main transistor. In the on time Ton of the MOSFET Q1, the current loop passes through a diode D1, the MOSFET Q1, and the inductor L1, and at this time, the inductor L1 stores energy. In the off time Toff of the MOSFET Q1, the current loop passes through the diode D1, the capacitor C, the MOSFET Q2, and the inductor L1, and at this time, the inductor L1 outputs energy.
The bridge arm connected to the inductor L1 is still taken as an example to illustrate the principle of a CRM control manner in the following. For the sake of simplicity, only a working principle in the positive half cycle of inputting the alternating current is introduced herein.
In the positive half cycle of inputting the alternating current, the MOSFET Q2 acts as the main transistor. In the on time Ton of the MOSFET Q2, the current loop passes through the inductor L1, the MOSFET Q2, and the diode D2, and in the off time Toff of the MOSFET Q2, the current loop passes through the inductor L1, the MOSFET Q1, the capacitor C, and the diode D2. At this time, the MOSFET Q1 acts as a synchronous rectifier transistor, and in this period of time, the MOSFET Q1 is driven, so that the MOSFET Q1 is turned on, and a current flowing through the inductor L1 drops linearly. When it is detected that the current of the inductor L1 drops near zero ampere (A), the MOSFET Q1 is turned off, so that the current continues to flow through the body diode of the MOSFET Q1. Because of a reverse recovery characteristic of the body diode of the MOSFET Q1, a certain reverse recovery current exists, and this reverse recovery current is used to conduct the body diode of the MOSFET Q2, thereby implementing zero voltage switching of the MOSFET Q2. FIG. 2 shows the current waveform (a triangular wave) of the inductor L1 and the drive voltage waveform (a square wave) of the MOSFET Q1. The principle in the negative half cycle of inputting the alternating current is similar to that in the positive half cycle of inputting the alternating current.
However, the reverse recovery current flowing through the body diode of the switching component (for example, a MOSFET) is uncontrollable, and the reverse recovery current changes with the input voltage and the load. Meanwhile, the reverse recovery current also influences a soft switching state of the switching component. In addition, in the case of inputting a high voltage, the inductor cannot obtain a negative current, and therefore, zero voltage switching of the switching component (that is, a MOSFET) cannot be implemented.